A redox flow cell is a type of secondary cell in which charging/discharging is performed using oxidation-reduction reaction by respectively circulating a positive electrode cell active material and a negative electrode cell active material in a positive electrode chamber and a negative electrode chamber using a pump. The positive electrode chamber and negative electrode chamber are obtained by dividing an electrolytic tank using a membrane that permits permeation of hydrogen ions but does not permit permeation of electrolytic ions. A large redox flow cell can be readily obtained, so that the redox flow cell has been expected as a secondary cell to store a large amount of electric power.
As the cell active materials for the redox flow cell, vanadium-based compounds have been widely used in recent years due to the following reasons: they provide excellent electromotive force and excellent cell capacity; regeneration thereof can be readily attained even if the positive electrode electrolyte and the negative electrode electrolyte are mixed with each other; and the like. During charging/discharging in a V—V-based redox flow cell employing such vanadium (V)-based compounds as the cell active materials, oxidation or reduction takes place between +4 valent V ions and +5 valent V ions in the positive electrode chamber and oxidation or reduction takes place between +2 valent V ions and +3 valent V ions in the negative electrode chamber.
In order to obtain excellent charging/discharging efficiency (high energy efficiency=high voltage efficiency×high current efficiency), a redox flow cell membrane is required to have a low membrane resistance (electric resistance during charging/discharging) and a property to prevent mixing of the electrolytes between the positive electrode chamber and the negative electrode chamber. Specifically, the redox flow cell membrane is required to have excellent permeability for hydrogen ions (H+; hydronium ions) and is also required to less likely permit permeation of electrolytic ions (excellent selective permeability). In view of this, for a redox flow cell membrane, an ion exchange membrane selectively permitting permeation of ions has been employed conventionally.
Further, the redox flow cell membrane is required to have excellent durability such that performance of the membrane is not decreased due to contact with the electrolytes for a long time as well as repeated charging/discharging. In particular, the V—V-based redox flow cell membrane is required to have a strong oxidation resistance because it is subjected to pentavalent V ions, which are strong in oxidation power.
As a cell membrane satisfying the above-described requirements, a polysulfone-based anion exchange membrane, a vinyl-based anion exchange membrane having a pyridinium group, or the like has been proposed (Patent Document 1 (Japanese Patent Laying-Open No. 2000-235849) and Patent Document 2 (Japanese Patent Laying-Open No. 10-162853)). Also proposed as such a cell membrane is an ion exchange membrane in which a resin composition, which includes an ion exchange resin and a copolymer obtained through hydrogenation treatment of a copolymer of conjugated diolefin and a styrene-based monomer, is adhered to a base material (Patent Document 2).
Known examples of a material of the base material of the cell membrane include porous polyvinyl chloride, polyolefin, polytetrafluoroethylene (PTFE), and the like (paragraph 0022 of Patent Document 2; claim 2 of Patent Document 1).
There has been also known a redox flow cell including an ion exchange membrane having a performance index advantageous to improve voltage efficiency and ampere-hour efficiency (Patent Document 3).